1. Field of the Invention
The present invention is directed to a system for producing a real-time video image from an x-ray image, of the type employing a CCD solid-state image sensor for converting light signals corresponding to the x-ray image into electrical signals.
2. Description of the Prior Art
Although CCD sensors are widely used in the production of images for display or storage, CCD sensor are limited in read-out speed. The cause lies in the frequency response of the output amplifier on the chip or in the limited shift speed of the charges in the registers of the CCD sensor.
In order to realize higher read-out speeds, CCD sensors that have two or four outputs, via which different portions of the image are simultaneously read out, are known.
A problem with such multiple output CCD sensors is that the respective amplifiers connected to each output unavoidably have characteristics that deviate slightly from one another (linearity error), so that the location where the separation of the sub-images lies in the CCD sensor is visible. This is an undesired effect that is of significance in diagnostic x-ray images because the demands made of amplitude resolution in, for example, digital radiography are extremely high.
A complicating factor when digitization is carried out is that analog-to-digital converters have additional linearity errors and long-term drift in the d.c. voltage level as well as in gain. These errors and additional influences of analog amplifiers that precede the sensor likewise lead to synchronization errors between the channels.
The errors can become especially undesirable when a steepening of the amplitude characteristic is required, for example logarithmizing for digital subtraction angiography.
As a consequence of the these problems and since digital corrections still require considerable processing times, it has heretofore been problematical to employ a CCD sensor with more than one output for applications sensitive to image quality, such as for high-resolution x-ray video imaging in a real-time mode. On the contrary, an operating mode had to be selected--insofar as possible--wherein only one output is employed. As a result, however, the maximum image frequency is reduced, thereby limiting the employment of such sensors in, for example, cardiology.